The present invention relates to a control apparatus for a voltage-controlled inverter which subjects an A.C. power source to variable-frequency and variable-voltage conversion to drive an induction motor at a variable speed.
Referring to the prior art apparatus shown in FIG. 1, numeral 1 designates the main inverter circuitry which subjects an A.C. power source to variable-voltage and variable-frequency conversion to drive an induction motor 2 at a variable speed. Drive circuitry 3 drives controllable elements within the main inverter circuitry. A current detector 4 detects the inverter output current. A first current level setting unit 5 sets a level for protecting the elements of the main inverter circuitry from overcurrents. Numeral 6 designates control circuitry which receives the set signal of the unit 5 and the current detection value of the detector 4, and which prepares a variable-frequency and variable-voltage control signal on the basis of a velocity command while monitoring that the inverter output current does not exceed the level set by the unit 5.
A conventional variable-frequency and variable-voltage control method employing the inverter apparatus of FIG. 1 and the relationships between the slip S of the induction motor 2 and the torque T and current I thereof will be described with reference to FIGS. 3(A) and 4(A). In general, in performing the variable-frequency and variable-voltage control, the voltage V/frequency F ratio-constant control method is employed in which, as illustrated in FIG. 3(A), the magnetic flux is held constant irrespective of the frequency so as to maintain the generated torque at a constant magnitude (in a lower frequency region, however, the voltage is set at a somewhat greater value in order to compensate the drop component ascribable to resistance). The frequency at this time is denoted by F.sub.1 and the voltage by V.sub.1, and the relationships of the slip S to the generated torque T and current I are illustrated in FIG. 4(A). The main circuit elements of the inverter apparatus are selected and determined to permit to flow therethrough enough current to generate the rated torque of the induction machine having the same capacity as that of the inverter. A current I.sub.o, which is set for protecting the elements, exists in a region less than a slip value S.sub.m1 at which a stall torque T.sub.m1 is generated. Usually, if the current is within the level I.sub.o, the motor will be stably operable without exceeding the slip value S.sub.m1.
The operation of the apparatus in FIG. 1 will be described with reference to FIG. 2-FIG. 4(B). As shown by way of example in FIG. 2, the main inverter circuitry 1 consists of a three-phase full-wave diode converter portion 11 which converts A.C. supply voltages into D.C., a smoothing capacitor which smooths the D.C. voltages, and a three-phase inverter portion 12 which is composed of transistors and diodes and which subjects the D.C. voltages to variable-frequency and variable-voltage conversion. A permissible current level is set by the first current level setting unit 5 in order to protect the semiconductor elements of the inverter portion 12 from overcurrents.
Upon receiving the velocity command, the control circuitry 6 determines on-off signals for the transistors of the inverter portion 12 on the basis of the received command in order to deliver a voltage and a frequency corresponding to the velocity command in accordance with a predetermined voltage/frequency ratio pattern. In consequence, the inverter performs the variable-voltage and variable-frequency conversion operation in response to the signal of the drive circuitry 3, and the induction motor 2 rotates at a speed at which the motor output torque determined from the present output voltage/frequency balances the load torque. At this time, the inverter output current determined from the output voltage/frequency and the rotating speed flows, and this current is detected by the current detector 4 and applied to the control circuitry 6. If the current detection signal is smaller than the first current level setting signal set by the level setting unit 5, the operation will be continued without any change. Ordinarily, the first current level setting value is greater than the required current at the rated torque of the induction motor corresponding to the inverter capacity, and the operation can be normally continued at or near the rated load.
In contrast, if the load torque is too great or the slip increases due to a rapid acceleration, the inverter output current will increase. If, at this time, the detected current value is greater than the first current level setting value, the inverter elements might be damaged. The increase of the slip is therefore suppressed by lowering the frequency or stopping its increase, to thus control the induction motor to operate at or below the first current level setting value. If, even with this measure, the output current remains above the first current level setting value without decreasing, the transistors will be turned "off" to stop the inverter and protect the elements.
With the conventional inverter controller operating as described above, with just a single current level being set to protect the elements, if the capacity of the induction motor is smaller than the inverter capacity, or the output voltage/frequency ratio is small in conformity with the load as illustrated in FIG. 3(B), or the output voltage/frequency ratio is small above a rated frequency for a constant output operation as illustrated in FIG. 3(C), then the first current setting level I.sub.o will exist at a point greater than a slip value S.sub.m2 generating a stall torque T.sub.m2 as illustrated in FIG. 4(B). In this case, even when the load has increased, the motor continues to operate with the voltage/frequency ratio remaining unchanged, until the current reaches I.sub.o. This has led to the disadvantage that the motor stalls due to its operation entering a slip region greater than the slip value S.sub.m2.